PROJECT SUMMARY/ABSTRACT Alzheimer's disease (AD) is a neurodegenerative disease that results in amyloid ? plaque deposition, neurofibrillary tangle (NFT) formation, and life-altering cognitive defects. The ?4 allelic variant of the apolipoprotein E (APOE) gene is the greatest human genetic AD risk factor and can modulate microglial function and neuroinflammation. Peripheral immune cells (e.g. monocytes) can infiltrate the CNS and significantly alter cognition and central nervous system (CNS) sequelae during amyloidosis. Interestingly, APOE4 has been shown to exacerbate peripheral inflammation following bacterial or viral mimetic stimulation. We hypothesize that monocytes expressing different human APOE isoforms will significantly modulate CNS sequelae during amyloid driven disease in the APP/PS1 mode. We further predict that APOE isoforms will alter peripheral and CNS infiltrating immunoprofiles during aging and drive significant changes in circulating peripheral blood mononuclear cell (PBMC) immune subsets and transcriptomes in aged humans and mice. To address this hypothesis, we will utilize APOE3-TR and APOE4-TR mice, which express human APOE, and clinical samples from ?3/?3 and ?3/?4 patients. To examine the role of APOE isoforms on peripheral monocytes and disease, we will adoptively transfer monocytes from APOE3, or APOE4 TR mice into APP/PS1 mice and their impact on cognition and pathology. We will assess multiple parameters including cognitive performance using contextual fear conditioning and morris water maze, immunofluorescence examining plaque deposition and microglial/astrocyte reactivity, flow cytometric analysis of CNS immune infiltrates, and RT-qPCR to examine cortical and hippocampal gene expression of proinflammatory and anti-inflammatory factors. Furthermore, to assess the impact of APOE isoforms on peripheral and central immune cells during aging, flow cytometric immunophenotyping of the blood, lymphoid organs, and brain parenchyma of young, middle-aged, and old mice APOE3 and APOE4 TR mice will be conducted. Importantly, human ?3/?3 and ?3/?4 peripheral blood mononuclear cells (PBMCs) and APOE3 and APOE4 TR murine PBMCs will be examined using single cell transcriptomics to assess immune subpopulations and transcriptome profiles. Our proposed work will be the first to define how human APOE isoforms impact the ability of peripherally derived monocytes to modulate cognition and CNS neuroinflammation, gliosis, and pathology during amyloidosis. Additionally, it will provide fundamental insights regarding how APOE isoforms and aging interact to shape peripheral immunity and CNS infiltrating cells, as well as cutting edge research on the impact of APOE isoforms on transcriptome profiles of human and murine PBMCs. This work will provide highly novel insights into how APOE isoforms impact peripheral immunity during aging and disease, and could provide the impetus to examine new therapeutic measures (e.g. CRISPR/Cas9 therapy) for management/alleviation of AD associated CNS sequelae.